Evaluation of Complexation Titrimetry
The scale of operations, accuracy, precision, sensitivity, time, and cost of methods involving complexation titrations are similar to those described earlier for acid–base titrimetric methods. Compared with acid–base titrations, however, complexation titrations are more selective. Despite the ability of EDTA to form strong complexes with virtually all metal ions, carefully controlling the pH at which the titration is carried out makes it possible to analyze samples containing two or more analytes (see Example 9.10). The reason that pH can be used to provide selectivity is easily appreciated by examining Figure 9.32. A titration of Ca2+ at a pH of 9 gives a dis- tinct break in the titration curve because the conditional formation constant (K´f = 2.6 x 109) is large enough to ensure that the reaction of Ca2+ and EDTA goes to completion. At a pH of 3, however, the conditional formation constant (K´f = 1.23) is so small that very little Ca2+ reacts with the EDTA.
Spectrophotometric titrations are particularly useful for the analysis of mixtures if a suitable difference in absorbance exists between the analytes and products, or titrant. For example, the analysis of a two-component mixture can be accomplished if there is a difference between the absorbance of the two metal–ligand complexes (Figure 9.33).
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